You should hire an attorney to help you. After you've retained an attorney discuss how to report the mother for assaulting you and discuss the evidence that you have to prove that your ex let you use the car. Depending on the strength of the evidence that you have, your attorney may advise you to tell the police your side of the story. Unless you have solid corroboration of the fact that she let you use the vehicle your attorney will probably advise you to exercise your right to remain silent.
You most definitely need the help of qualified legal counsel. Do not post any additional information on this public website or elsewhere on the Internet.
They may be looking at you for a number of different charges including theft of a motor vehicle, which is a felony in Pennsylvania. However, they may also want to speak to you concerning the alleged assault, but if they already spoke to her I highly doubt that was her story. It would be very wise for you to retain local counsel to begin prompt dialogue with the police, act as a buffer for now between you and police, and also advise you on what you should do depending on how it pans out. If the police are looking for you they will eventually find you. Right now it is too early to tell what the outcome will be, but there is major benefit to retaining counsel prior to having police contact, as opposed to after, when many times it is too late. Best advice - hire local counsel promptly.
I agree with my colleagues, you need an attorney, say no more on a public forum. Best of luck.
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